In a book entitled "LED Circuits and Projects," by Forrest M. Mims, III, published in 1973 by Howard W. Sams and Co., Inc., it is proposed that the same electrooptic device be used as both a source and a detector of optical wave energy. In particular, it is noted that an LED used as a detector is sensitive to the same wavelength emitted by an LED composed of the same semiconductor materials when used as a source. Accordingly, a single LED at each end of a communication link can be used alternately as a source and as a detector to provide two-way (i.e., duplex) communication along a single optical transmission line without the need for optical couplers or branching networks. A system of this kind is particularly attractive for use in the subscriber loop network of a telephone system as an inexpensive means of providing two-way communications between the telephone company central office and its local subscribers. A problem with such an arrangement, however, is the reverse Rayleigh scattering inherent in the optical fiber used as the transmission link between office and subscriber.
As disclosed in U.S. Pat. No. 3,717,769, when an optical wave propagates along a fiber, some fraction of the light is reflected back towards the source because of the phenomenon known as Rayleigh scattering. The above-cited patent was particularly concerned with the process called "double-reverse scattering" wherein the scattered energy, trapped within the fiber, is reverse-scattered a second time so that it propagates once again in its original direction towards a remote receiver. Because of the delay, the scattered energy arrives after the original signal, giving rise to an appreciable interference signal.
Of concern here, is the effect upon the system when the single reverse-scattered energy arrixes delayed at the source which is now operating in its dual role as a receiver. In this latter case, a similar interference effect is produced by the single reverse-scattered energy as it interferes with the desired signal that originated at the remote station. The problem is similar to that produced by echoes which occur in conventional communication systems. However, in such systems the echo is discretely produced at the remote station and the echo canceller is located at the remote station. See, for example, U.S. Pat. No. 3,508,017. By contrast, Rayleigh scattering is a distributed phenomenon which occurs along the entire length of the transmission path.